1. Field of the Invention
The present invention provides a system and method for performing automatic frequency control (AFC) in a Frequency-shift-key (FSK) data transmission system that allows a receiver to be used that has a bandwidth that approaches the minimum bandwidth for a given data transmission rate and, as a consequence, allows a substantial signal-to-noise ratio (SNR) to be realized in the signal detected by the receiver.
2. Description of the Related Art
A method that is widely used to transmit binary data is the frequency-shift-key (FSK) method. In the FSK method, an FSK transmitter modulates the frequency of a carrier signal between two predetermined frequencies according to the logical state, logical "0" or logical "1", of a binary data signal to produce an FSK signal. For convenience, the portions of the FSK signal corresponding to a logical "0" and a logical "1" in the binary data signal are hereinafter referred to as a SPACE and a MARK, respectively.
As in all data transmission systems, a minimum bandwidth is required in an FSK data transmission system in order to accurately transmit binary data. The minimum bandwidth required in the receiver of an FSK data transmission system is defined by the following equation: EQU FSK.sub.min bw =B+.DELTA.f (1)
where B is the baseband bandwidth, which is the data transmission rate or frequency of the binary data, and .DELTA.f is the frequency deviation. If the bandwidth of the receiver in an FSK data transmission system is less than the minimum bandwidth, then the receiver cannot reliably recover the binary data in the FSK signal output by the transmitter. If, on the other hand, the bandwidth in the receiver of an FSK data transmission system is greater than the minimum bandwidth, then noise can adversely affect the performance of the system by reducing the signal-to-noise ratio (SNR) of the binary data signal detected or recovered by the receiver. An FSK data transmission system where the bandwidth of the receiver is at or near the minimum bandwidth is less susceptible to noise and, as a consequence, the signal detected by the receiver has a higher SNR. Based on the foregoing, it can be seen that there is an optimum bandwidth for an FSK data transmission system, the minimum bandwidth set forth in equation (1), that is broad enough to adequately transmit binary data but narrow enough to substantially reduce or minimize the adverse effects of noise.
The bandwidth of most FSK receivers is established in the design of the intermediate frequency (i.f.) filter or filters that process the signal output by a mixer, a device that converts the frequency of the signal received by the receiver to a lower frequency. Consequently, in designing an FSK data transmission system, the required data transmission rate is determined, the minimum necessary bandwidth is calculated using equation (1), and an i.f. filter or series of filters is designed that has at least the minimum bandwidth. Conventionally, the midpoint of the i.f. filter bandwidth is termed the i.f. frequency.
The signal output by the mixer is termed the i.f. signal and has a frequency that is equal to the difference between the frequency of the received signal and the frequency of the signal output by a local voltage controlled oscillator (VCO). To avoid confusion between the frequency of the i.f. signal output by the mixer and the i.f. frequency of the i.f. filter, the i.f. frequency of the i.f. filter is hereinafter referred to as the center frequency of the i.f. filter. The frequency of the i.f. signal can be determined by the following equation: EQU f.sub.IF =f.sub.TX -f.sub.VCO ( 2)
where f.sub.IF is the frequency of the i.f. signal output by the mixer, f.sub.TX is the frequency of the signal output by the transmitter, and f.sub.VCO is the frequency of the signal output by the VCO.
To recover the binary data in an FSK data transmission system, the receiver must be tuned to the transmitter by adjusting the frequency of the signal output by the VCO so that the frequency spectrum of the i.f. signal output by the mixer is substantially symmetrical about the center frequency of the i.f. filter. If the receiver is tuned, then the SNR of the signal detected by the receiver will increase as the bandwidth of the receiver approaches the minimum bandwidth necessary for a given data transmission rate.
A high SNR is extremely desirable in situations where data must be transmitted and/or received in environments where the transmitted signal is subjected to a high degree of noise. To achieve a high SNR, the receiver must have a bandwidth at or near the minimum bandwidth and the receiver must be precisely tuned. This allows the receiver to recover the binary data while at the same time substantially reducing the adverse effects of noise. Unfortunately, the frequency of the carrier signal output by the transmitter, the frequency of the signal output by the VCO, or both are likely to drift due to changes in temperature and the like. To compensate for drift in the transmitter carrier frequency, the frequency of the signal output by the VCO, or both, the receiver is typically equipped with automatic frequency control (AFC) circuitry that automatically tunes the receiver to the transmitter, i.e., adjusts the frequency of the signal output by the VCO such that the spectrum of the signal output by the mixer is substantially symmetrical about the center frequency of the i.f. filter.
One method of achieving automatic frequency control is to continuously tune the receiver using a feedback signal that reflects the difference in frequency between the signal being output by the transmitter and the frequency to which the receiver is tuned. More specifically, this method involves continuously comparing the frequency of the signal being output by the transmitter with the frequency to which the receiver is tuned to generate a difference signal, continuously averaging the difference signal over a short period of time, and then tuning the receiver such that the average of the difference signal, which is typically obtained at the output of the receiver's detector, tends toward zero. This method works well with an analog data signal, like speech or music, where the average value of the data signal is close to zero and, as a consequence, the average frequency of the signal output by the transmitter is at or near the carrier frequency. Since the average frequency of the signal output by the transmitter is at or near the carrier frequency, the frequency of the signal output by the VCO can be adjusted by the AFC so that the spectrum of the i.f. signal is substantially symmetrical about the center frequency of the i.f. filter and, as a consequence, substantially all of the data can be recovered. However, this method does not work well with binary data signals that often exhibit a dc component, due to a number of consecutive logical "1"'s or logical "0"'s over a defined time period, because the average frequency of the signal output by the transmitter over the defined time period is greater than or less than the carrier frequency by an amount that reflects the dc component of the binary signal. Due to the dc component, the AFC adjusts the frequency of the signal output by the VCO such that the spectrum of the i.f. signal output by the mixer is not substantially symmetrical about the center frequency of the i.f. filter and, as a consequence, adversely affects the recovery of the modulation signal by the receiver. Typically, this problem is addressed by adding "balancing" SPACEs or MARKs to eliminate the dc component in the binary data signal. This solution, however, reduces the transmission rate of the meaningful data and in so doing substantially nullifies the benefits of using a receiver with a bandwidth at or near the minimum bandwidth, i.e., a high SNR.
Another method of accomplishing automatic frequency control that is used in packet FSK data transmission systems where data is generally transmitted in bursts or packets of predetermined lengths involves transmitting an unmodulated carrier signal at the beginning of each packet in what is typically known as a preamble. The receiver, upon receiving the preamble, activates a feedback loop that utilizes the difference in the frequency of the carrier signal output by the transmitter and the frequency to which the receiver is tuned to generate a difference signal that is used to tune the receiver during the transmission of the data contained in the remainder of the packet. An example of this method of obtaining automatic frequency control in a phase-shift-key data transmission system is shown in U.S. Pat. No. 4,651,104, which issued on Mar. 17, 1987 to Miyo for a "Frequency Converter with Automatic Frequency Control". While this method of automatic frequency control allows a receiver to be utilized that has a bandwidth that approaches the minimum necessary bandwidth for a given data transmission rate and achieve a substantial signal to noise ratio, it has several drawbacks. Among the drawbacks, this method requires that the transmitter incorporate additional circuitry to generate the unmodulated carrier frequency that is transmitted during the preamble of a packet. Typically, FSK data transmission systems only incorporate the circuitry required to generate the SPACE signal and the MARK signal. Consequently, the need to include circuitry for transmitting the unmodulated carrier signal significantly adds to the cost of FSK data transmission systems that employ this method of achieving automatic frequency control. Another drawback associated with this method of achieving automatic frequency control in FSK data transmission systems is that the transmitter must be aligned such that the frequency of the SPACE signal and the frequency of the MARK signal are symmetrical about the frequency of the carrier signal. Otherwise, this method of AFC may detune the receiver and, in so doing, reduce the reliability of the FSK data transmission system. Moreover, the need to accurately align the transmitter adds significant manufacturing costs to an FSK data transmission system.
Based on the foregoing, there is a need for a system and method of achieving automatic frequency control in an FSK data transmission system that allows a receiver with a bandwidth that approaches the minimum bandwidth for a given data transmission rate to be utilized, realizes a high SNR, and also addresses the failings in the known art discussed hereinabove. Specifically, there is a need for a system and method of providing AFC in an FSK data transmission system that allows a receiver with a bandwidth that approaches the minimum required bandwidth for a given data transmission rate to be utilized and produces a high SNR but does not require any circuitry in the transmitter to generate an unmodulated carrier signal. Moreover, there is a need for a system and method of providing automatic frequency control in an FSK data transmission system that allows a receiver with a bandwidth that comes near to the minimum bandwidth required for a defined data transmission rate to be used and produces a high SNR but does not require the frequencies associated with the SPACE and MARK portions of the signal output by the transmitter to be symmetrical about the frequency of the carrier signal. Additionally, there is a need for an automatic frequency control system and method in an FSK data transmission system that permits a receiver with a bandwidth that tends toward the minimum bandwidth required for the data transmission rate to be employed and provides a high SNR without reducing the data transmission rate of the meaningful data by using "balancing" SPACES and MARKS to compensate for the dc component typically associated with the binary data signals employed in an FSK data transmission system.